Mass spectrometers are instruments that are used to determine the chemical structures of molecules. In these instruments, molecules become positively or negatively charged in an ionization source and the masses of the resultant ions are determined in vacuum by a mass analyzer that measures their mass/charge (m/z) ratio. Mass analyzers come in a variety of types, including magnetic field (B), combined (double-focusing) electrical (E) and magnetic field (B), quadrupole (Q), ion cyclotron resonance (ICR), quadrupole ion storage trap, and time-of-flight (TOF) mass analyzers. Double focusing instruments include Nier-Johnson and Mattauch-Herzog configurations in both forward (EB) and reversed geometry (BE). In addition, two or more mass analyzers may be combined in a single instrument to form a tandem mass spectrometer (MS/MS, MS/MS/MS, etc.). The most common MS/MS instruments are four sector instruments (EBEB or BEEB), triple quadrupoles (QQQ), and hybrid instruments (EBQQ or BEQQ).
The mass/charge ratio measured for a molecular ion is used to determine the molecular weight of a compound. In addition, molecular ions may dissociate at specific chemical bonds to form fragment ions. Mass/charge ratios of these fragment ions are used to elucidate the chemical structure of the molecule. Tandem mass spectrometers have a particular advantage for structural analysis in that the first mass analyzer (MS1) can be used to measure and select molecular ions from a mixture of molecules, while the second mass analyzer (MS2) can be used to record the structural fragments. In tandem instruments, a means is provided to induce fragmentation in the region between the two mass analyzers. The most common method employs a collision chamber filled with an inert gas, and is known as collision induced dissociation CID. Such collisions can be carried out at high (5-10 keV) or low (10-100 eV) kinetic energies, or may involve specific chemical (ion-molecule) reactions. Fragmentation may also be induced using laser beams (photodissociation), electron beams (electron induced dissociation), or through collisions with surfaces (surface induced dissociation). While the four sector, triple quadrupole and hybrid instruments are commercially available, tandem mass spectrometers utilizing time-of-flight analysis for either one or both of the mass analyzers are not commercially available.
In a time-of-flight mass spectrometer, molecular and fragment ions formed in the source are accelerated to a kinetic energy: EQU eV=mv.sup.2 /2
determined by the potential difference (V) across the source/accelerating region. These ions enter a field-free drift region of length L with velocities (v) that are inversely proportional to the square root of their mass/charge ratios (m/e): EQU v=(2 eV/m).sup.1/2
The time required for a particular ion to traverse the drift region is directly proportional to the square root of the mass/charge ratio: EQU t=L(m/2 eV).sup.1/2
Conversely, mass/charge ratios of ions can be determined from their flight times according to the equation: EQU m/e=at.sup.2 +b
where a and b are experimental constants determined from the flight times of two ions of known mass/charge.
Generally, time-of-flight mass spectrometers have very limited mass resolution. This arises because there may be uncertainties in the time that the ions were formed (time distriuton), in their location in the accelerating field at the time they were formed (spatial distriution), and in their initial kinetic energy distributions prior to acceleration (energy distriuton).
The first commercially successful time-of-flight mass spectrometer was based on an instrument described by Wiley and McLaren in 1955 (Wiley , W. C.; McLaren, I. H., Rev. Sci. Instrumen. 26 1150 (1955)). That instrument utilized electron impact (E1) ionization (which is limited to volatile samples) and a method for spatial and energy focusing known as: time-lag focusing. In brief, molecules are first ionized by a pulsed (1-5 microsecond) electron beam. Spatial focusing was accomplished using multiple-stage acceleration of the ions. In the first stage, a low voltage (-150 V) drawout pulse is applied to the source region that compensates for ions formed at different locations, while the second (and other) stages complete the acceleration of the ions to their final kinetic energy (-3 keV). A short time-delay (1-7 microseconds) between the ionization and drawout pulses compensates for different initial kinetic energies of the ions, and is designed to improve mass resolution. Because this method required a very fast (40 ns) rise time pulse in the source region, it was convenient to place the ion source at ground potential, while the drift region floats at -3 kV. The instrument was commercialized by Bendix Corporation as the model MA-2, and later by CVC Products (Rochester, N.Y.) as the model CVC-2000 mass spectrometer. The instrument has a practical mass range of 400 daltons and a mass resolution of 1/300, and is still commercially available.
There have been a number of variations on this instrument. Muga (TOFTEC, Gainsville) has described a velocity compaction technique for improving the mass resolution (Muga velocity compaction). Chatfield et al. (Chatfield FT-TOF) described a method for frequency modulation of gates placed at either end of the flight tube, and fourier transformation to the time domain to obtain mass spectra. This method was designed to improve the duty cycle.
Cotter et al. (VanBreemen, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion. Phys. 49 (1983) 35.; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; Olthoff, J. K.; Lys, I: Demirev, P.: Cotter, R. J., Anal. Instrumen. 16 (1987) 93) modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. This group also constructed a pulsed liquid secondary time-of-flight mass spectrometer (liquid SIMS-TOF) utilizing a pulsed (1-5 microsecond) beam of 5 keV cesium ions, a liquid sample matrix, a symmetric push/pull arrangement for pulsed ion extraction (Olthoff, J. K.; Honovich, J. P.; Cotter, Anal. Chem. 59 (1987) 999-1002.; Olthoff, J. K. ; Cotter, R. J., Nucl. Instrum. Meth Phys. Res. B-26 (1987) 566-570). In both of these instruments, the time delay range between ion formation and extraction was extended to 5-50 microseconds, and was used to permit metastable fragmentation of large molecules prior to extraction from the source. This in turn reveals more structural information in the mass spectra.
The plasma desorption technique introduced by Macfarlane and Torgerson in 1974 (Marfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res. Commun. 60 (1974) 616.) formed ions on a planar surface placed at a voltage of 20 kV. Since there are no spatial uncertainties, ions are accelerated promptly to their final kinetic energies toward a parallel, grounded extraction grid, and then travel through a grounded drift region. High voltages are used, since mass resolution is proportional to U.sub.o /eV, where the initial kinetic energy, U.sub.03 is of the order of a few electron volts. Plasma desorption mass spectrometers have been constructed at Rockefeller (Chait, B. T.; Field, F. H., J. Amer. Chem. Soc. 106 (1984) 193), Orsay (LeBeyec, Y.; Della Negra, S.; Deprun, C.; Vigny, P.; Ginot, Y. M., Rev. Phys. Appl 15 (1980) 1631), Paris (Viari, A.; Ballini, J. P.; Vigny, P.; Shire, D.; Dousset, P., Biomed. Environ. Mass Spectrom, 14 (1987) 83), Upsalla (Hakansson, P.; Sundqvist. B., Radiat. Eff. 61 (1982) 179) and Darmstadt (Becker, O.; Furstenau, N.; Krueger, F. R.; Weiss, G.; Wein, K., Nucl. Instrum. Methods 139 (1976) 195). A plasma desorption time-of-flight mass spectrometer has been commercialized by BIO-ION Nordic (Upsalla, Sweden). Plasma desorption utilizes primary ion particles with kinetic energies in the MeV range to induce desorption/ionization. A similar instrument was constructed at Manitobe (Chain, B. T.; Standing, K. G., Int. J. Mass Spectrom. Ion Phys. 40 (1981) 185) using primary ions in the keV range, but has not been commercialized.
Matrix-assisted laser desorption, introduced by Tanaka et al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151) and by Karas and Hellenkamp (Karas, M.; Hillenkamp, F., Anal Chem. 60 (1988) 2299) utilizes time-of-flight mass spectrometry to measure the molecular weights of proteins in excess of 100,000 daltons. An instrument constructed at Rockefeller (Beavis, R. C.; Chait, B. T., Rapid Commun. Mass Spectrom. 3 (1989) 233) has been commercialized by VESTEC (Houston, Tex.), and employs prompt two-stage extraction of ions to an energy of 30 keV.
Time-of-flight instruments with a constant extraction field have also been utilized with multi-photon ionization, using short pulse lasers.
The instruments described thus far are linear time-of-flights, that is: there is no additional focusing after the ions are accelerated and allowed to enter the drift region. Two approaches to additional energy focusing have been utilized: those that reflect the ions back through the drift region, and those which pass the ion beam through an electrostatic energy filter.
The reflectron (or ion mirror) was first described by Mamyrin (Mamyrim, B. A.; Karatajev, V. J.; Shmikk, D. V.; Zagulin, V. A., Sov Phys., JETP 37 (1973) 45). At the end of the drift region, ions enter a retarding field from which they are reflected back through the drift region at a slight angle. Improved mass resolution results from the fact that ions with larger kinetic energies must penetrate the reflecting field more deeply before being turned around. These faster ions then catch up with the slower ions at the detector and are focused. Reflectrons were used on the laser microprobe instrument introduced by Hillenkamp et al. (Hillenkamp, F.; Kaufmann, R.; Nitsche, R.; Unsold, E., Appl. Phys. 8 (1975) 341) and commercialized by Leybold Hereaus as the LAMMA (LAser Microprobe Mass Analyzer). A similar instrument was also commercialized by Cambridge Instruments as the IA (Laser Ionization Mass Analyzer). Benninghoven (Benninghoven reflectron) has described a SIS (secondary ion mass spectrometer) instrument that also utilizes a reflectron, and is currently being commercialized by Leybold Hereaus. A reflecting SIS instrument has also been constructed by Standing (Standing, K. G.; Beavis, R.; Bollbach, G.; Ens, W.; Lafortune, F.; Main. D.; Schueler, B.; Tang, X.; Westmore, J. B., Anal. Instrumen. 16 (1987) 173).
LeBeyec (Della-Negra, S.; Leybeyec, Y., in Ion Formation from Organic Solis IFOS III, ed by A. Benninghoven, pp 42-45, Springer-Verlag, Berlin (1986)) described a coaxial reflectron time-of-flight that reflects ions along the same path in the drift tube as the incoming ions, and records their arrival times on a channelplate detector with a centered hole that allows passage of the initial (unreflected) beam. This geometry was also utilized by Tanaka et al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T., Rapid Commun. Mass Spectrom. 2 (1988) 151) for matrix assisted laser desorption. Schlag et al. (Grotemeyer, J.; Schlag, E. W., Org. Mass Spectrom. 22 (1987) 758) have used a reflectron on a two-laser instrument. The first laser is used to ablate solid samples, while the second laser forms ions by multiphoton ionization. This instrument is currently available from Bruker. Wollnik et al. ( Grix., R.; Kutscher, R.; Li, G.; Gruner, U.; Wollnik, H., Rapid Commun. Mass Spectrom. 2 (1988) 83) have described the use of reflectrons in combination with pulsed ion extraction, and achieved mass resolutions as high as 1/20,000 for small ions produced by electron impact ionization.
An alternative to reflectrons is the passage of ions through an electrostatic energy filter, similar to that used in double-focusing sector instruments. This approach was first described by Poschenroeder (Poschenroeder, W., Int. J. Mass Spectrom. Ion Phys 6 (1971) 413). Sakurai et al. (Sakuri, T.; Fujita, Y.; Matsuo, T.; Matsuda, H.; Katakuse, I., Int. J. Mass Spectrom. Ion Processes 66 (1985) 283) have developed a time-of-flight instrument employing four electrostatic energy analyzers (ESA) in the time-of-flight path. At Michigan State, an instrument known as the ETOF was described that utilizes a standard ESA in the TOF analyzer (Michigan ETOF).
Lebeyec et al. (Della-Negra, S.; Lebeyec, Y., in Ion Formation from Organic Solis IFOS III, ed. by A. Benninghoven, pp 42-45, Springer-Verlag, Berlin (1986)) have described a technique known as correlated reflex spectra, which can provide information on the fragment ion arising from a selected molecular ion. In this technique, the neutral species arising from fragmentation in the flight tube are recorded by a detector behind the reflectron at the same flight time as their parent masses. Reflected ions are registered only when a neutral species is recorded within a preselected time window. Thus, the resultant spectra provide fragment ion (structural) information for a particular molecular ion. This technique has also been utilized by Standing (Standing, K. G.; Beavis, R.; Bollbach, G.; Ens, W.; Lafortune, F.; Main. D.; Schueler, B.; Tang, X.; Westmore, J. B., Anal. Instrumen. 16 (1987) 173).
Although time-of-flight mass spectrometers do not scan the mass range, but record ions of all masses following each ionization event, this mode of operation has some analogy with the linked scans obtained on double-focusing sector instruments. In both instruments, MS/MS information is obtained at the expense of high resolution. In addition correlated reflex spectra can be obtained only on instruments which record single ions on each time-of-flight cycle, and are therefore not compatible with methods (such as laser desorption) which produce high ion currents following each laser pulse. Thus, a true tandem time-of-flight configuration with high resolution would consist of two reflecting mass analyzers, separated by a collision chamber.
New ionization techniques, such as plasma desorption (MacFarlane, R. D.; Skowronski, R. P.; Torgerson, D. F.; Biocem. Bios. Res. Commun. 60 (1974) 616), laser desorption (VanBreemen, R. B.; Snow, M.; Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Van der Peyl, G.J.Q.; Isa, K.; Haverkamp, J.; Kistemaker, P. G.; Org. Mass Spectrom. 16 (1981) 416), fast atom bombardment (Barber, M.; Bordoli, R. S.; Sedwick, R. D.; Tyler, A. N., J. Chem. Soc., Chem Commun. (1981) 325-326) and electrospray (Meng, C. K.; Mann, M. Fenn, J. B., Z. Pys. D10 (1988) 361), have made it possible to examine the chemical structures of proteins and peptides, glycopeptides, glycolipids and other biological compound without chemical derivatization. The molecular weights of intact proteins can be determined using matrix-assisted laser desorption on a time-of-flight mass spectrometer or electrospray ionization. For more detailed structural analysis, proteins are generally cleaved chemically using CNBr or enzymatically using trypsin or other proteases. The resultant fragments, depending upon size, can be mapped using matrix-assisted laser desorption, plasma desorption or fast atom bombardment. In this case, the mixture of peptide fragments (digest) is examined directly resulting in a mass spectrum with a collection of molecular ions corresponding to the masses of each of the peptides. Finally, the amino acid sequences of the individual peptides which make up the whole protein can be determined by fractionation of the digest, followed by mass spectral analysis of each peptide to observe fragment ions that correspond to its sequence.
It is the sequencing of peptides for which tandem mass spectrometry has its major advantages. Generally, most of the new ionization techniques are successful in producing intact molecular ions, but not in producing fragmentation. In the tandem instrument the first mass analyzer passes molecular ions corresponding to the peptide of interest. These ions are fragmented in a collision chamber, and their products extracted and focused into the second mass analyzer which records a fragment ion (or sequence) spectrum.